1. Field of the Invention
The present invention relates to a method for etching nitride, and more particularly to a wet-etching method for nitride enhanced by utilizing UV light.
2. Description of Prior Art
Using III-V group semiconductors to manufacture light-emitting devices is a well-known prior art. The procedure of manufacturing light-emitting devices is similar to that of manufacturing Si-based devices, which comprises epitaxy, photolithography, etching, diffusion and coating, etc. However, due to the difference of semiconductor materials, each procedure mentioned above needs to be modified when manufacturing a device with different semiconductor materials. Particularly in the etching step, different materials result in very different ways of etching. For example, the photoelectrochemical (PEC) etching technology disclosed in J. Electrochem. Soc. 138, pp. 1174-1185 (1991) by M. N. Rubert et al., which incorporates the effect of visible light and chemical matter, has been widely applied to the etching of narrow energy band III-V group semiconductor materials such as GaAs or InP.
However, only three prior arts are disclosed applying the photoelectrochemical etching technology to wide energy band semiconductor materials such as GaN (gallium nitride). The first one is "Room-temperature photoenhanced wet etching of GaN," Appl. Phys. Lett. 68, pp. 1531-1533 (1996) disclosed by M. S. Minsky et al., which employs a He-Cd laser (570 mW/cm.sup.2, 325 nm) while etching under no bias voltage. The second one is "Photoassisted anodic etching of GaN," J. Electrochem. Soc. 144, L8-L11(1997) disclosed by H. Lu et al., which applies mercury lamps of 60 mW/cm.sup.2 at 365 nm and 150 mW/cm.sup.2 at 405 nm to the etching under bias. The third one is "Broad-area photoelectroch. etching of GaN," Elec. Lett. 33, pp. 245-246 (1997) disclosed by Youtsey et al., which uses unfiltered mercury lamps of 6.4 mW/cm.sup.2 at 320 nm, 7.4 mW/cm.sup.2 at 365 nm and 13.2 mW/cm.sup.2 at 405 nm as light sources to work in the etching under no bias voltage.
In the prior arts above, the drawbacks of Minskey et al. to are that the effective area of the laser spot working on GaN is too small, normally only 1 mm.sup.2, and that the mode distribution of the incident light after passing through the glass wall of the vessel containing etching liquid is not spatially uniform. These drawbacks cause inferior yield and degrade performance during the fabrication of light-emitting devices.
Regarding to Lu et al., the wavelengths of the incident light used in etching are 365 nm and 405 nm, both longer than the energy band absorption of GaN that is 3.4 eV, i.e. 364.7 nm. In this case, the function of the bias voltage is similar to the anode electrolysis of electrochemistry. The mercury lamp of 365 nm or 405 nm is only used to provide heat during the etching process.
As to Youtsey et al., the manufacturing process suffers from variation in etching depth. Taking a sample of 0.5.times.1 cm.sup.2 for example, the variance of etching depth is 20.about.30% when the impurity concentration is about 10.sup.18 cm.sup.-3, and up to 80% when the impurity concentration is about 10.sup.16 cm.sup.-3.
Each of the three prior arts has the following drawbacks: the roughness of the etching surface is around 100 nm under the conditions in each prior art. However, such a roughness will cause remarkable optical loss and degrade the efficiency of light-emitting devices for laser diodes having a wavelength less than 450 nm.